1. Field of the Invention
Embodiments of the present invention generally relate to a silicon controlled rectifier (SCR) layout within an integrated circuit (IC) and, more particularly, to an SCR layout that maximizes the width/area ratio of an SCR layout.
2. Description of the Related Art
Silicon-controlled rectifiers (SCRs) find widespread use in many applications where a switched resistive path is desired. For example, SCRs find particular use in electrostatic discharge (ESD) circuits or devices that are placed between an output or input pad of an IC and ground. The ESD circuits provide a high resistance path through an SCR when the circuit being protected is operating normally, and provides a low resistance path from the pad to ground when an ESD event occurs. The ESD event triggers the SCR into a low resistance state that shunts ESD generated current from the pad to ground. In this manner, the circuit being protected is not damaged by the ESD event.
There are many variations of ESD circuits that utilize SCRs in this manner. Commonly assigned U.S. Pat. Nos. 6,768,616, 6,791,122, 6,850,397 and 6,909,149, which are incorporated herein by reference, describe a number of such ESD circuits.
In conventional SCRs, the amount of current that can be handled by an SCR is proportional to the width of the SCR. FIG. 1 depicts a simplified top plan view of a 1-sided SCR 100. The SCR comprises an anode 102 and a cathode 104 that abut one another along an active area 106. As is well known in the art, the SCR 100 is generally formed of a PNPN device, the details of which are well-known and not shown in this simplified view. The SCR 100 has a length L and a width W. When the SCR 100 is not triggered, the SCR has a high resistive path from the anode 102 to cathode 104. Conversely, when the SCR 100 is triggered in a low resistance state, current I flows from the anode 102 to the cathode 104 along the width W of the SCR 100, i.e., along the active area 106. The wider the SCR width W, the wider the active area 106 and the higher the current handling capability of the SCR. Since the depth of the SCR 100 is fixed by the IC manufacturing parameters, the current handling ability of the SCR is solely controlled by the SCR width W. In an ESD circuit, an ESD circuit designer selects an SCR width that provides a suitable level of ESD protection against an ESD event.
To increase the current handling capability without increasing the physical width of the SCR, a 2-sided SCR may be used. FIG. 2 depicts a simplified top plan view of a 2-sided SCR 200 comprising a first anode 202, a cathode 204 and a second anode 206. The two anodes 202 and 206 abut the cathode 204 along active areas 208 and 210. When the SCR is active, current flows from both anodes 202, 206 into the cathode 204. As such, the effective width of the SCR is double that of the 1-sided SCR. As with the 1-sided SCR, an ESD designer controls the current handling level by adjusting the physical width W of the SCR 200.
If the region of the circuit in which the SCR is formed has limited space for the SCR width, the designer may not be able to achieve the width of the SCR that is necessary for the desired current handling capability. The result will be a compromised design.
Therefore, there is a need in the art for increasing the current handling capability of an SCR without increasing the physical width of the SCR, i.e., increasing the current handling for a given SCR area.